As recording media for recording and reproducing information, various kinds of recording media have been proposed. As for an optical disc of such recording media, optical discs of various standards have been proposed and widely spread depending on various uses.
For example, a CD-DA (Compact Disc Digital Audio) disc is an optical disc on which digital audio data such as music is recorded. A DVD (Digital Versatile Disc)-video disc serves to record digital video data such as videos or images. As optical discs of a new generation, optical discs capable of recording HDTV (High Diffusion Television) data have been studied. The above-described CD-DA discs or the DVD-video discs respectively employ optical beam whose wavelength is 780 nm or 650 nm to reproduce the data thereof. On the other hand, the optical discs of the new generation respectively employ optical beam whose wavelength is, for instance, 400 to 410 nm and an objective lens whose numerical aperture NA is 0.85 to record or reproduce data.
The above-described CD-DA discs have been developed for the purpose of recording the audio data. Then, a standard concerning a CD-ROM disc for recording data that can be processed by a computer is formed. The CD-DA discs and the CD-ROM discs are optical discs dedicated for reproducing. On the other hand, a standard concerning a CD-R (Compact Disc Recordable) disc or a CD-RW (Compact Disc Rewritable) disc capable of recording data has been also further formed and put into practical use.
The DVD-video discs have been developed for the purpose of recording digital video data such as videos or images. A standard concerning a DVD-ROM disc for recording data that can be processed by a computer is also formed like the CD-ROM disc. The DVD video discs or the DVD-ROM discs are optical discs dedicated for reproducing. On the other hand, a standard concerning a DVD-R (DVD Recordable) disc or a DVD-RW(DVD Rewritable) disc capable of recording data is formed. As for the DVD discs capable of data, various kinds of formats have been proposed and commercially merchandised.
These discs of a CD system such as the CD-DA discs or the CD-ROM discs are different from the discs of a DVD system such as the DVD-video discs or the DVD-ROM discs in modulation system of data to be recorded or error correction encoding system or the like. However, any of these discs has the diameter of 12 cm or 8 cm and substantially has the thickness of 1.2 mm. A drive that can record or reproduce data in a single recording and/or reproducing part, that is, a recording and/or reproducing device has been commercially merchandised for the CD discs and the DVD discs having the common size as described above. The recording and/or reproducing device that makes it possible to commonly use the CD discs and the DVD discs can record or reproduce data respectively on a plurality of kinds of optical discs having different standards as mentioned above.
A dual layer optical disc having two recording layers laminated has been also proposed by paying attention to a point that the CD disc and the DVD disc have the diameter of 8 cm or 12 cm and the thickness of about 1.2 mm. However, an optical beam incident on the recording layer of the two recording layers of the dual layer optical disc remote from a plane of incidence of an optical beam for recording and reproducing penetrates the recording layer near the plane of incidence. Thus, the strength of the optical beam is lowered. The optical beam reflected on the remote recording layer penetrates again the near recording layer. Accordingly, the remote recording layer may not be possibly irradiated with the optical beam having an adequate strength necessary for recording or the optical beam having an adequate strength may not be possibly obtained upon reproducing. The transmittance of the recording layer near the plane of incidence of the optical beam is raised so that a recording or a reproducing on the recording layer remote from the plane of incidence of the optical beam are properly performed. In this case, reflected light from the recording layer near the plane of incidence cannot be inconveniently sufficiently obtained upon reproducing of the recording layer near the plane of incidence.